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Florida Fish and Wildlife Conservation Commission officials said 538 of the marine mammals were found dead in the state’s waterways last year.

Michelle Kerr, a spokeswoman for the Florida Fish and Wildlife Conservation Commission's research institute in St. Petersburg, said red tide algae blooms contributed to a higher number of reported deaths in 2017. Red tide is the condition in which toxic algae bloom, releasing a toxin in the water that kills wildlife.

“Red tide is a form of algae that contains what’s called a brevetoxin. Manatees can breathe this in or eat plants that absorb it,” said marine biologist Patrick Rose, who is also a member of the Save the Manatee Club. “It doesn’t initially kill manatees, (but) rather causes muscle spasms or even paralysis that can ultimately lead to the manatee drowning.”

He said researchers believe 60 manatee deaths last year were related to red tide.

Rose said the main factor in manatee deaths varies from year to year. In 2013, red tide was the main cause, and in 2010, it was cold weather.

Statewide, 106 manatee deaths were attributed to watercraft collisions, according to numbers from the FWC. Kerr said watercraft collisions account for about 20 percent of manatee deaths over the past five years.